The geometry of various components within semiconductor devices are important to the performance of such devices. For example, ultra-shallow source/drain regions are desired and a MOS transistor transconductance relies upon the width-to-length ratio (W/L) of the device. In addition to the importance of the device geometries, the doping concentration within the regions (i.e., the doping concentration profiles) are relevant to the performance of the device.
In light of the above concerns regarding the geometry and doping concentrations of regions within semiconductor structures, various methods exist and are used to determine the junction depths and doping concentration of various regions to thereby monitor the semiconductor process. The various methods include sheet resistance measurements, junction depth measurements and doping profile measurements.
A four-point probe technique is used to measure the sheet resistance Rs of a film or doped region. The sheet resistance Rs of a film (which in this instance is the wafer) is determined as follows in conjunction with prior art FIG. 1. The resistance (R) of a rectangular shaped film of length (L), width (W) and thickness (t) is given by the equation: EQU R=.rho.L/tW,
wherein .rho. equals the resistivity of the film, which is unique for a given material, and is measured in .OMEGA.-cm. If the length L is equal to the width W, then the rectangle is a square and the equation reduces to: EQU R=.rho./t=Rs,
wherein Rs is the sheet resistance in .OMEGA./square and is independent of the size of the square (but does depend on the resistivity of the material and the thickness of the film). Therefore the resistivity .rho. and the sheet resistance Rs are distinct parameters that are related by the above equation.
The four-point probe method is illustrated in prior art FIG. 2. If the sample film may be approximated as semi-infinite with respect to the spacings (s) between the four probes (which are spaced apart substantially equally from one another), the current (I) is driven as shown and the voltage drop (V.sub.1 -V.sub.2) is measured across the remaining probes as illustrated in prior art FIG. 2. The sheet resistance may then be calculated according to the following equation: EQU Rs=(V.sub.1 -V.sub.2)(2 .pi.s)/It.
To prevent erroneous readings using the four point technique (e.g., due to thermoelectric heating and cooling) the measurement is often performed with current forced in both directions and the two readings are averaged. Further, the test is often performed at several current levels (i.e., I.sub.1, I.sub.2, etc.), until the proper current level is found. For example, if the current is too low, the forward and reverse current readings will substantially differ and if the current is too high, I.sup.2 R heating will result in the measured reading drifting over time. Although the American Society for Testing and Materials (ASTM) provides standards which recommend current levels for a given resistivity range, one may still need to vary the current about the recommended value to achieve the optimum current for an accurate measurement which undesirably takes extra time.
The sheet resistance Rs and the resistivity .rho. are found using the measured results and the equation V/I(2 .pi.s), wherein s is the probe spacing. The above equation, however, is only accurate if the sample is semi-infinite with respect to the probe spacings, which is often not an accurate assumption. Thus, the sheet resistance is typically calculated by the relation: EQU Rs=(V/I)F.sub.1,
wherein F.sub.1 is a correction factor which is a function of the average probe distance s and the wafer diameter D (i.e., F.sub.1 =f(s/D)). Since the four-point probe technique uses a correction factor and occupies a space of at least 3s due to the four probes, the readings are not totally accurate and further represent merely an average resistivity within the region of 3s.
Several prior art methods are used to measure a junction depth (x.sub.j) of various regions such as source/drain regions, n-wells, p-wells, etc. These prior art methods include, for example: (1) angle-lap and stain, (2) groove and stain, and (3) transmission electron microscopy (TEM). The angle-lap and stain method involves grinding a wafer to form an angle or beveled surface at an angle .theta. as illustrated in prior art FIG. 3 using, for example, a slurry having a lapping compound. After grinding the wafer, a staining solution is used to delineate the n-type and p-type areas. The staining solution includes, for example, copper sulphate (CuSO.sub.4) and the stained region is then subjected to an intense light source which causes the stained junction to become forward biased. The copper atoms then plate onto the n-type region which thereby delineates the regions and allows a lateral measurement (x) of the stained region, as illustrated in FIG. 3. The junction depth (x.sub.j) is then measured according to the formula: EQU x.sub.j =x tan .theta..
The angle-lap and stain method, however, suffers from problems because large sources of error may occur from uncertainty of the lapping angle. This is usually addressed by subsequently measuring the formed lapping angle using a laser reflection system which undesirably increases the complexity and cost of the measurement. Another problem with the angle-lap and stain method is that if the CuSO.sub.4 concentration is too high, excessive copper plating occurs which results in poor junction delineation. In addition, if the concentration is too low, little plating will occur, especially on lightly doped n-type regions.
The groove and stain method uses a rotating ball or cylinder having, for example, a diamond grit which contacts the wafer and cuts a groove in the silicon which is sufficiently deep to expose the junction to be measured, as illustrated in prior art FIG. 4. After the groove is formed, it is stained in a manner similar to that discussed supra and measurements (e.g., x and y of FIG. 4) are taken, and the junction depth (x.sub.j) is determined according to the following formula: EQU x.sub.j =xy/2R,
wherein R is the radius of the arc formed by the rotating ball or cylinder. The groove and stain method suffers the same staining limitations as discussed above with respect to the angle-lap and stain procedure. In addition, the groove and stain method suffers from errors due to the groove not being exactly cylindrical and also from difficulties in determining the edge of the groove (for measuring x and y of FIG. 4) when viewed under a microscope.
Another method of measuring the junction depth involves transmission electron microscopy (TEM). The TEM method involves applying a wet etch type solution such as 0.5% HF in HNO.sub.3 to selectively remove the n-type material and subsequently analyzing the etched result using the TEM to measure the junction depth (x.sub.j). TEM, however, requires a significant amount of work to properly prepare the sample, including ion milling, etc. which makes implementation of TEM burdensome.
As discussed above, each of the above junction depth measurements has limitations. In addition, each of the above test procedures is destructive and cannot be performed in-line (i.e., within the semiconductor manufacturing process) and is thus undesirable.
Several prior art doping concentration profile measurement methods exist, including: (1) the capacitance-voltage (C-V) technique, (2) spreading resistance measurements, and (3) secondary ion mass spectroscopy (SIMS). The capacitance voltage technique is based on the reverse biased capacitance of a p-n junction. The technique requires the formation of a shallow p+ or n+ region over the surface of the region of interest to generate the p-n junction and the capacitance is measured while varying a reverse bias voltage. The doping concentration "n" is then determined according to the following equation: EQU C(V)=(q.epsilon..sub.s n/2).sup.1/2 [V.sub.bi.+-.V.sub.R -(2kT/q).sup.1/2
wherein .epsilon..sub.s is the permittivity of silicon, V.sub.bi is the built-in potential of the junction and V.sub.R is the reverse bias voltage. The depth at which the concentration "n" is measured is obtained from the value of the reverse bias voltage V.sub.R. The capacitance-voltage technique is limited for extremely shallow junctions since the technique is unable to determine the concentration at the surface (within the zero bias depletion region) and also has difficulty measuring abrupt profiles.
The spreading resistance measurement utilizes the spreading resistance (i.e., the resistance associated with the divergence of current when a probe is placed on the doped region). To make the spreading resistance measurement, a known current is applied between the two probes and the voltage drop is measured across the probes (R.sub.SR =V/I). The value of the measured spreading resistance is related to the resistivity by the following: EQU .rho.=2R.sub.SR a,
wherein "a" is an empirical value which is related to the probes taking the measurement. The spreading resistance technique is then used to determine the doping concentration by angle lapping the processed wafer as discussed above and making spreading resistance measurements along the length of the angle. Because the angle of the cut (or ground) wafer is known, the depth may be easily determined by using the distance between the probe and the edge of the wafer. The spreading resistance technique suffers from the same limitations as the angle-lap and stain technique since extra work must be performed to accurately determine the angle for subsequent measurements.
Secondary ion mass spectroscopy (SIMS) is another method used to measure doping profiles. SIMS involves bombarding the processed wafer with atoms (e.g., oxygen atoms) which collide with atoms on the surface which cause the atoms to be ejected from the surface (i.e., sputtering). During the energy transfer process, a small fraction of the ejected atoms leave as either positively or negatively charged ions which are collected by the mass spectrometer. The ion yield of the wafer is measured and a linear dependence between the ion yield and the doping concentration is used to determine the profile. Again, since the processed wafer is sputtered, the test is destructive and cannot be performed in-line.
In light of the above limitations in the prior art, it would be desirable to have a method of determining the doping type and the doping concentration of a processed semiconductor wafer that provides a more accurate and convenient resistivity reading without destroying the region being tested.